Along with the fast development of OTN technology in recent years, International Telecommunications Union-Telecommunication Standardization Sector (ITU-T) has made a series of OTN recommendations, ITU-T G.709, G.798, G.87X, and commercial applications of OTN products are entering the industry, among which the recommendation of G.709 put forward in February 2001 is significant and has laid the technical foundation of optical networking. The key element of the G.709 is the technique of digital wrapper, which defines a special frame format so as to encapsulate a customer signal into a payload unit of a frame while providing overhead (OH) bytes at the head of the frame used for Operation Administration Maintenance and Provision (OAM&P) and providing forward error correction (FEC) bytes at the tail of the frame.
The standard frame format adopted by the digital wrapper is shown in FIG. 1. It is a frame format of 4 rows and 4080 columns, the 16 columns at the head of the frame are the overhead bytes, the 255 columns at the tail of the frame are FEC bytes, and the 3808 columns in the middle are payload.
Among the overhead bytes at the head of the frame, columns 1-8 of the 1st row is a Frame Alignment Signal (FAS), columns 9-14 are the overhead bytes of the kth type of Optical Channel Transport Unit-k (OTUk), wherein different values of k correspond to transmission modes with different rates, columns 1-14 of the 2nd to 4th row are the overhead bytes of Optical Channel Data Unit-k (ODUk), and columns 15 and 16 are the overhead bytes of Optical Channel Payload Unit-k (OPUk).
OTUk overhead bytes provide a monitoring function for the status of transmitted signal between the 3R (reamplification, reshaping, and retiming) regenerated nodes in the OTN, including 3 portions: the overhead bytes for Section Monitoring (SM), the overhead bytes for inter-terminal General Communication Channel (GCC) 0, and reservation (RES) bytes.
ODUk overhead bytes provide concatenation monitoring, point-to-point channel monitoring and provide customer signal adaptation via the OPUk. The ODUk provides plenty of overhead bytes (columns 1-14 of rows 2-4) to perform the above functions, including bytes of Path Monitoring (PM) overhead, Tandem Connection Monitoring (TCM) overhead, GCC 1 and GCC 2 overhead, Auto-Protection Switching/Protection Control Channel (APS/PCC) overhead, Fault Type Fault Location (FTFL) information, and Experiment (EXP) overhead.
The OPUK consists of payloads of mapped customer signals and related overhead, the overhead bytes include Payload Structure Identifier (PSI), Payload Type (PT), Reserved (RES) byte, and Mapping Specific Overhead etc.
At present, there are three methods for mapping the customer signal to the OTN, which are as follows:
(1) mapping signals with Constant Bit Rate (CBR) 2G5, CBR10G, and CBR40G to the OPUk: constant bit rate signals with CBR2G5—2488320 kbit/s±20 ppm, e.g. Synchronous Transfer Module (STM)—16; constant bit rate signals with CBR10G—9953280 kbit/s±20 ppm, e.g. STM—64; constant bit rate signals withCBR40G—39813120 kbit/s±20 ppm, e.g. STM—256. Two modes, an asynchronous mode and a bit synchronous mode, may be adopted in mapping. In the asynchronous mode, a local clock not associated with the customer signal is used with the positive/negative/zero code adjustification mechanism. In the bit synchronous mode, the clock extracted from the customer signal is used.
(2) mapping signals of Asynchronous Transfer Mode (ATM) to the OPUk: mapping the signals to the OPUk by multiplexing ATM Information Elements (IE) to constant bit streams matching the payload volume of the OPUk, the rate is adjusted by inserting idle IE or discarding IE during multiplexing operations. Information of ATM IE should be scrambled before mapping.
(3) mapping General Framing Procedure (GFP ) frame signals to the OPUk: mapping GFP frames to continuous bit streams matching the OPUK by inserting idle frames during packaging, and scrambling is also conducted in this process. Other signals may be mapped to the OPUk as well, such as the customer signal, test signal, and common customer bit stream signal.